Laser-MOPA with burst-mode control

Abstract

A laser master-oscillator power-amplifier (MOPA) is operated to provide successive bursts of ultrashort pulses. The pulse-bursts are selected by an optical modulator from a pulse train delivered by the master oscillator prior to amplification in the power amplifier. The optical modulator has a selectively variable transmission specified by an analog voltage signal having a stepped waveform. The voltage signal is delivered by a sequentially-switched parallel switch-array connected in parallel with a parallel DAC having multiple parallel DC voltage outputs corresponding to steps of the stepped waveform.

What is claimed is: 
     
       1. Optical apparatus, comprising:
 a pulsed seed laser generating a train of seed pulses having a pulse repetition frequency; 
 an optical modulator arranged to receive the train of seed pulses from the pulsed seed laser and to form a burst of seed pulses by selecting a plurality of consecutive seed pulses from the train of seed pulses, the optical modulator having an optical transmission specified by an analog modulation signal having a stepped waveform; 
 an optical amplifier arranged to receive the burst of seed pulses from the optical modulator and to amplify the seed pulses; 
 a parallel digital-to-analog converter having a plurality of output channels providing direct-current voltages that correspond to voltage steps of the stepped waveform; and 
 a parallel switch-array connected in parallel with the output channels of the parallel digital-to-analog converter and arranged to deliver the analog modulation signal to the optical modulator; 
 wherein the stepped waveform is formed by sequential switching of the parallel switch-array between the output channels of the parallel digital-to-analog converter. 
 
     
     
       2. The apparatus of  claim 1 , wherein the pulsed seed laser includes a mode-locked resonator. 
     
     
       3. The apparatus of  claim 1 , wherein the pulsed seed laser includes a directly-driven semiconductor laser. 
     
     
       4. The apparatus of  claim 1 , wherein the optical modulator is an acousto-optic modulator, the optical transmission of the acousto-optic modulator being about proportional to the analog modulator signal. 
     
     
       5. The apparatus of  claim 1 , wherein the optical modulator is an electro-optical modulator. 
     
     
       6. The apparatus of  claim 1 , wherein the optical amplifier is a fiber amplifier. 
     
     
       7. The apparatus of  claim 1 , wherein the parallel digital-to-analog converter and parallel switch-array are elements of a controller, the controller further comprising a computer, the computer sending a digital signal to control the direct-current voltages on the output channels of the parallel digital-to-analog converter in response to user input. 
     
     
       8. The apparatus of  claim 7 , wherein the controller further comprises a field-programmable gate-array, the computer communicating a required start time of the burst of seed pulses, a required duration of the burst of seed pules, and a desired phase shift for operation of the parallel switch-array with respect to the arrival of seed pulses at the optical modulator. 
     
     
       9. The apparatus of  claim 7 , wherein the controller further comprises a counter-encoder, the counter-encoder communicating an encoded digital signal to a decoder that is connected to the parallel switch-array, the encoded digital signal representing a number of switches of the parallel switch-array to be operated in sequence. 
     
     
       10. The apparatus of  claim 9 , wherein the encoded digital signal further represents a switching order. 
     
     
       11. The apparatus of  claim 9 , wherein the controller further comprises a phase-shift clock, the phase-shift clock advancing or retarding the encoded digital signal to achieve the desired phase shift. 
     
     
       12. The apparatus of  claim 1 , wherein the voltage steps of the stepped waveform cause stepwise changes in the optical transmission of the optical modulator, these stepwise changes in the optical transmission being synchronized with respect to the arrival of seed pulses at the optical modulator, the stepwise changes in the optical transmission being out of phase with the arrival of seed pulses at the optical modulator. 
     
     
       13. The apparatus of  claim 12 , wherein a phase shift between the stepwise changes in optical transmission and the arrival of seed pulses at the optical modulator is about one half of the separation between seed pulses. 
     
     
       14. The apparatus of  claim 1 , further including an array of low-pass filters and a low-pass filter, the filter array located between and in parallel connection with the parallel digital-to-analog converter and the parallel switch-array, the low-pass filter located between and in serial connection with the parallel switch-array and the optical modulator. 
     
     
       15. The apparatus of  claim 1 , further including a digital signal that initiates and terminates operation of the optical modulator. 
     
     
       16. The apparatus of  claim 1 , wherein the analog modulation signal has a progressively increasing waveform, the optical amplifier delivering a burst of amplified seed pulses having about equal amplitude. 
     
     
       17. The apparatus of  claim 1 , wherein the direct-current voltages are selected such that the optical amplifier delivers a burst of amplified seed pulses having amplitudes that progressively increase and then progressively decrease. 
     
     
       18. The apparatus of  claim 1 , wherein each direct-current voltage determines the amplitude of one amplified seed pulse in the burst thereof. 
     
     
       19. The apparatus of  claim 1 , wherein a change in amplitude profile of the amplified seed pulses is achieved by commanding a new set of direct-current voltages on the output channels of the parallel digital-to-analog converter. 
     
     
       20. The apparatus of  claim 1 , wherein a change in amplitude profile of the amplified seed pulses is achieved by commanding a different order of sequential switching.